Composite glass and resin optical element with an aspheric surface

ABSTRACT

In order to provide an optical element with an aspheric surface, as well as a process by which the optical element can be mass-produced with high efficiency and consistent precision, the improved optical element with an aspheric surface includes a glass substrate and an overlying light-transmissive resin layer worked to have an aspheric surface. This optical element can be produced by molding a light-transmissive resin composition as it is cured between a glass substrate and a mold having an aspheric surfaced shape. The preferred light-transmissive resin has a Rockwell hardness of 40-80 on the M scale at -40° C. to 60° C.

BACKGROUND OF THE INVENTION

This application is based on and claims priority from Japanese PatentApplications No. Hei 3-145599 filed May 21, 1991 and No. Hei 3-203483filed Jul. 19, 1991, the disclosures of which are incorporated byreference herein.

The present invention relates to an optical element with an asphericsurface and a process for producing the same.

Optical elements with an aspheric surface such as aspheric lenses havebeen proposed for correcting the aberrations that will develop inspherical lenses and, hence, they are used as eyepieces and highphotographic lenses. Lenses are manufactured from glass or plastics andin order to work glass to have an aspheric surface, grinding andpolishing operations are necessary for each lens. Hence, aspheric glasslenses have the disadvantage of being unsuitable for large-scaleproduction and having only limited consistency in the precision of lensmachining. Under these circumstances, aspheric glass lenses find utilityonly in specialty optical elements. In contrast, aspheric plastic lensescan be shaped with molds and hence are highly suitable for large-scaleproduction. However, they do not have high heat and dimensionalstability and can only be used in applications that do not require veryhigh precision.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical element withan aspheric surface of high precision that is improved in both thermaland dimensional stability.

Another object of the present invention is to provide a process by whichoptical elements with an aspheric surface of the type described abovecan be mass-produced with high efficiency and consistent precision.

These and other objects of the present invention can be achieved notonly by using a glass substrate having high heat and dimensionalstability but also by composing this glass substrate with an opticalresin in such a way that the aspheric surface of the resin is molded tobecome integral with the glass substrate.

In its first aspect, the present invention provides an optical elementwith an aspheric surface that comprises a glass substrate and anoverlying light-transmissive resin layer worked to have an asphericsurface.

In its second aspect, the present invention provides a process forproducing an optical element with an aspheric surface that comprisesmolding a light-transmissive resin composition as it is curved between aglass substrate and a mold having an aspheric surfaced shape.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a side view of a spherical glass lens;

FIG. 2 is a diagram showing how an aspheric lens is produced by theprocess of the present invention; and

FIG. 3 is a cross-sectional view of the aspheric lens of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, it is preferred that the glass substrate hasbeen preliminarily worked to have a spherical surface. In other words,the aspheric surface that is to correct the aberrations that willdevelop in the glass substrate which is preliminarily worked to have aspherical surface is preferably formed of a light-transmissive resinlayer. It should, however, be noted that the present invention is by nomean limited to this particular embodiment.

An known thermosetting or uv curable resin can be used as thelight-transmissive resin and they may be exemplified by epoxy resins,acrylate resins, methacrylate resins, styrene resins, urethane resinsand polyethylene glycol derivated resins. Compared to uv curable resins,thermosetting resins have a very good balance in the transmittance ofvisible light, are stable in transmittance, refractive index anddispersion, permit uniform thermal polymerization and are capable offorming thick layers consistently. Further, thermosetting resins absorbless ultraviolet radiation and hence are resistant to yellowing. Inaddition, thermosetting resins have no limitations on the material ofwhich the substrate glass can be made. Because of these and many otheradvantages of thermosetting resins, suitable resin materials arepreferably selected in consideration of various factors such as theobject of use of the optical element with an aspheric surface to bemanufactured and the thickness of the resin layer.

While the light-transmissive resin layer can be formed of various typesof resins as described above, preferred resin layers are such that theyare capable of deforming by themselves to absorb the stress that willdevelop upon joining with the glass substrate, that will inherentlyproduce a small stress upon joining, that will shrink by a small amountupon curing and that have a small coefficient of thermal expansion.Further, in consideration of the need to form an anti-reflection coatover the light-transmissive resin layer by evaporation of an inorganicmaterial, it is preferred to select a suitable resin material from amongthose which will deform thermally at temperatures of 100° C. and above.

Among the resin materials that satisfy the conditions described above,epoxy resins are particularly preferred. Advantageous epoxy resins arethose of bisphenol A type, bisphenol AD type and bisphenol F type, whichare cured with an acid anhydride, an amine or any other curing agents.Acid anhydrides that can be used as curing agents includehexahydrophthalic anhydride and methyl hexahydrophthalic anhydride, andamines that can be used as curing agents include aliphatic polyamines,polyaminoamides, aromatic diamines, alicyclic diamines and imidazoles.Other curing agents that can be used include phenolic resins, aminoresins, mercaptan compounds, dicyanodiamides and Lewis acid complexcompounds.

It is particularly preferred that the resin layer in the optical elementwith an aspheric surface in accordance with the present invention has aRockwell hardness of 40-80 on the M scale at -40° C. to 60° C. If itsRockwell hardness on the M scale is less than 40, the resin layer isundesirably soft and it is not only poor in chemical resistance but alsolow in thermal deformation temperature, thus making it difficult to forman anti-reflection coat by evaporation of an inorganic material. If theRockwell hardness on the M scale exceeds 80, the resin layer is so hardthat it will either separate from the substrate glass or destroy it uponapplication of stress.

To produce an optical element with an aspheric surface according to thepresent invention, a light-transmissive resin composition is molded asit is cured between the glass substrate described above and a moldhaving an aspheric surfaced shape. The light-transmissive resincomposition is a feed composition that is capable of forming alight-transmissive resin and contains not only a resin component such asthe monomer, oligomer or a curing agent that is necessary for eachproduction but also additives such as a solvent, a polymerizationinitiator and an antioxidant.

The light-transmissive resin layer may be formed directly on the glasssubstrate to produce a composite optical element with an asphericsurface that has good adhesion between the substrate and the resinlayer. The adhesion between the two members can be further enhanced bypreliminarily forming a layer of silane coupling agent on the glasssubstrate. Any known silane coupling agent may be used and examplesinclude silane compounds such as γ-glycidoxytrimethoxysilane,γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane,γ-glycidoxypropyldimethoxysilane,γ-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,γ-methacryloxypropyltrimethoxysilane, andγ-methacryloxypropylmethyl-dimethoxysilane, as well as the products oftheir hydrolysis. These silane coupling agents may be used either ontheir own or as admixtures.

The present invention is described below more specifically Withreference to the accompanying drawings. FIG. 1 is a side view of a glasslens 1 worked to have a spherical surface on both sides; the surface onone side has a radius of curvature of R₁ and the surface on the otherside has a radius of curvature of R₂.

To provide an aspheric surface on the side of glass lens 1 which has thecurvature radius of R₂, a mold 2 is provided that has a desired asphericmolding surface 2A as shown in FIG. 2. Then, a light-transmissive resincomposition 3 is placed on the side of glass lens 1 which has thecurvature radius of R₂, brought into registry with the aspheric surface2A of the mold 2 and polymerized to cure, thereby forming alight-transmissive resin layer 3A. Subsequently, the assembly is takenout of the mold to give a composite aspheric lens 4 which, as shown inFIG. 3, has an aspheric surface 2A made integral with the sphericalglass lens 1.

The drawings show the case where an aspheric surface is provided on aconcave side of the glass lens. If one wants to provide an asphericsurface on a convex side of the glass lens, he may place thelight-transmissive resin composition on a mold having a correspondingaspheric (concave) surface and the glass lens is then placed on thatconcave surface, followed by polymerization and curing of thelight-transmissive resin composition.

EXAMPLES OF THE INVENTION

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting. In the examples, a spherical glass lens made of BK7 was usedas a substrate; it had a diameter of 20 mm, with R₁ =28 mm and R₂ =55mm, and its thickness was 3 mm in the center.

Example 1

A hundred parts by weight of a silane coupling agent(γ-glycidoxypropyltrimethoxysilane) and 30 parts by weight of 0.1N HClwere mixed and hydrolyzed for 8 h. The resulting hydrolyzate was coatedon the side of the spherical glass lens having the curvature radius R₂and subsequently dried at 120° C. for 1 h to form a layer of silanecoupling agent. In the next place, a resin composition containing 100parts by weight of EPIKOTE 828 (the trade name of a bisphenol A typeepoxy resin produced by Yuka-Shell Epoxy Co., Ltd.) and 70 parts byweight of hexahydrophthalic anhydride (curing agent) was metered in agiven amount and dripped on the layer of silane coupling agent. Theresin coating was then brought into registry with a separately preparedmold (26 mmφ) having an aspheric surface and heated at 100° C. for 2 hso that it would cure to form a resin layer having a central thicknessof 50 μm and a maximum resin thickness of 250 μm, whereby a compositeaspheric lens was produced.

Example 2

A hundred parts by weight of γ-glycidoxypropyltriethoxysilane and 30parts by weight of 0.1N HCl were mixed and hydrolyzed for 8 h. Theresulting hydrolyzate was coated on the side of the spherical glass lenshaving the curvature radius R₂ and subsequently dried at 120° C. for 1 hto form a layer of silane coupling agent. In the next place, a resincomposition containing 100 parts by weight of EPIKOTE 828 (the tradename of a bisphenol A type epoxy resin produced by Yuka-Shell Epoxy Co.,Ltd.) and 90 parts by weight of methyl hexahyrophthalic anhydride(curing agent) was metered in a given amount and dripped on the layer ofsilane coupling agent. The resin coating was then brought into registrywith a separately prepared mold (26 mmφ) having an aspheric surface andheated first at 60° C. for 1 h, then at 120° C. for 2 h so that it wouldcure to form a resin layer having a central thickness of 100 μm and amaximum resin thickness of 600 μm, whereby a composite aspheric lens wasproduced.

Example 3

A hundred parts by weight of γ-glycidoxypropyltriethoxysilane and 30parts by weight of 0.1 HCl were mixed and the mixture was hydrolyzedwith stirring for 8 h. The resulting hydrolyzate was coated on the sideof the spherical glass lens having the curvature radius R₂ andsubsequently dried at 120° C. for 2 h to form a layer of silane couplingagent. In the next place, a resin composition containing 100 parts byweight of ECOBOND 45 CLEAR (the trade name of an epoxy resin produced byGrace Japan K.K.) and 100 parts by weight of ECOBOND 15 CLEAR (the tradename of a polyamine based curing agent produced by Grace Japan K.K.) wasmetered in a given amount and dripped on the layer of silane couplingagent. The resin coating was then brought into registry with aseparately prepared mold (26 mmφ) having an aspheric surface,polymerized at 25° C. for 8 h so that it would cure to form a resinlayer having a central thickness of 100 μm and a maximum resin thicknessof 600 μm, whereby a composite aspheric lens was produced.

Comparative Example 1

A hundred parts by weight of γ-glycidoxyproplytriethoxysilane and 30parts by weight of 0.1N HCl were mixed and the mixture was hydrolyzedwith stirring for 8 h. The resulting hydrolyzate was coated on the sideof the spherical glass lens having the curvature radius R₂ andsubsequently dried at 120° C. for 1 h to form a layer of silane couplingagent. In the next place, 50 parts by weight of methoxytetraethyleneglycol methacrylate, 50 parts by weight of methoxypolyethylene glycol400 methacrylate and 2 parts by weight of diethoxyacetophenone(photopolymerization initiator) were mixed and dripped on the layer ofsilane coupling agent. The resin coating was brought into registry witha separately prepared mold (26 mmφ) having an aspheric surface andexposed to a uv radiation (360 nm) at an intensity of 100 mJ/cm² so thatit would cure to form a resin layer having a central thickness of 50 μmand a maximum resin thickness of 250 μm, whereby a composite asphericlens was produced.

Comparative Example 2

A hundred parts by weight of γ-glycidoxytrimethoxysilane and 30 parts byweight of 0.1N HCl were mixed and the mixture was hydrolyzed withstirring for 8 h. The resulting hydrolyzate was coated on the side ofthe spherical glass lens having the curvature radius R₂ and subsequentlydried at 120° C. for 2 h to form a layer of silane coupling agent. Inthe next place, a resin composition having 100 parts by weight of methylmethacrylate mixed with 0.5 part by weight of benzoyl peroxide wasmetered in a given amount and dripped on the layer of silane couplingagent. The resin coating was then brought into registry with aseparately prepared mold (26 mmφ) having an aspheric surface,polymerized at 25° C. for 8 h so that it would cure to form a resinlayer having a central thickness of 100 μm and a maximum resin thicknessof 600 μm, whereby a composite aspheric lens was produced.

TESTS Rockwell hardness

The resins used in Examples 1-3 and Comparative Examples 1 and 2 wereshaped into test specimens in disk form that were ca. 5-10 mm thick andwhich had a diameter of ca. 20 mm. The Rockwell hardness of eachspecimen on the M scale was measured and the results are shown in Table1 below.

Heat cycle test

Ten samples were prepared for each of the composite aspheric lensesfabricated in Examples 1-3 and Comparative Examples 1 and 2 and theywere subjected to five heat cycles each consisting of cooling at -40° C.for 20 min, holding at room temperature for 10 min and heating at 60° C.for 20 min. Thereafter, the samples were visually checked for theseparation of the resin layer, as well as for the breakage of lens. Theresults are also shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                            Evaluation                                                                    of composite                                                                  aspheric lens                                                                   Separation                                                     Rockwell hardness                                                                            of resin  Breakage                                      Run No.  -40° C.                                                                         20° C.                                                                         60° C.                                                                       layer   of lens                               ______________________________________                                        Example                                                                       1        80       78      75    none    none                                  2        79       70      67    none    none                                  3        75       69      64    none    none                                  Comparative                                                                   Example                                                                       1        92       90      85    separation                                                                            none                                                                  occurred in                                                                   several                                                                       samples                                       2        93       90      85    separation                                                                            breakage                                                              occurred                                                                              occurred                                                              in all  in all                                                                samples samples                               ______________________________________                                    

In accordance with the present invention, optical elements with anaspheric surface of high precision that are improved in both thermal anddimensional stability can be mass-produced with high efficiency andconsistent precision. Resins that are selected to have a hardness in thespecified range are capable of absorbing by themselves the stress thatdevelops upon either thermal deformation or polymerization shrinkage;therefore, the optical elements with an aspheric surface that areproduced by the present invention have extremely high thermal anddimensional stability.

What is claimed is:
 1. An optical element with an aspheric surfacecomprising:a glass substrate and an overlying light-transmissive resinlayer, said resin layer having an aspheric surface, and a Rockwellhardness which is in the range of 40-80 on the M scale at -40° C. to 60°C.
 2. The optical element according to claim 1 wherein said glasssubstrate has a spherical surface.
 3. The optical element according toclaim 2 further comprising a layer of silane coupling agent between theglass substrate and the resin layer.
 4. The optical element according toclaim 1 wherein said resin layer is made of a uv curable resin or athermosetting resin.
 5. The optical element according to claim 4 furthercomprising a layer of silane coupling agent between the glass substrateand the resin layer.
 6. The optical element according to claim 1 furthercomprising a layer of silane coupling agent between the glass substrateand the resin layer.